I can’t offer a solution to this, but…
I offer 365 as a service. I don’t get involved in it, beyond taking a cut, I pass all 365 work to the company who supply it (they also look after my servers etc.)
Just recently we had an issue with a client not getting emails sent via forms on the site, not a Foundry site. Regular emails worked fine, and on testing the forms sent emails to my Gmail email no problem, but not the email account on 365.
After some digging, we discovered that one of the settings in the DNS/Nameserver (not sure which, not my area of expertise) was incorrect. Once fixed emails started to land in the 365 inbox. Prior to this fix, the emails were routing to my catchall box.
So, the takeaway was 365 requires quite a lot of settings to be 100% correct at the domain end of things to work, and even the professionals get it wrong from time to time. If you want to check if this is your issue, check your catchall.